Cochlear Radiation Dose Does Not Affect Hearing Preservation after Gamma Knife Radiosurgery in Those without Vestibular Schwannoma

Background: Radiation dose to the cochlea has been identified as a key factor in hearing preservation following stereotactic radiosurgery for vestibular schwannoma (VS). There has been little data on hearing outcomes following radiosurgery in patients with non-VS tumors located near and/or extending to the internal auditory canal (IAC). The relationship of radiation dose to the cochlea on hearing preservation in this subset of patients remains unclear. Most studies have utilized MRI-based modiolar point dose and have determined critical cutoff points between 3 to 5.3 Gy to be statistically significantly associated with loss of serviceable hearing after SRS for sporadic VS. This study was performed to assess hearing outcome for non-VS tumors receiving greater than 5 Gy modiolar point dose to the cochlea during gamma knife radiosurgery.

Methods: Patients with serviceable pre-treatment hearing who underwent SRS for non-VS tumors extending near and/or to the IAC between 2008 and 2012 using Gamma Knife Perfexion were reviewed. Patients who received a point-dose greater than 5 Gy to the cochlear modiolus were selected and studied. Data including radiosurgery treatment plans, tumor characteristics, pre- and post-treatment pure tone average, speech discrimination scores, and American Academy of Otolaryngology-Head and Neck Surgery hearing class were collected.

Results: A total of 14 patients were identified meeting study criteria. Five patients (36%) had cerebellopontine angle meningiomas, 4 patients (28%) had petroclival meningiomas, and 5 patients (36%) harbored glomus jugulare tumors. The mean duration of audiometric follow-up after SRS was 20.5 months (range 6.0 to 46.0, median 18.5 months). Serviceable hearing was maintained in 11 patients (78.5%) at last audiometric followup. The mean point dose to the cochlear modiolus was 8.3 Gy (range 5.4 to 19.2 Gy, median 7.1 Gy). Patients with preserved serviceable hearing had a mean cochlear modiolar point dose of 8.7 Gy versus 6.8 Gy in those who developed non-serviceable hearing. Tumor control rate was 100% during the followup period.

Conclusions: The hearing preservation rate after radiosurgery for non-VS tumors was 78.5%, despite considerable radiation dose to the ipsilateral cochlea. This would suggest that there may be other more significant tumor-related factors, beyond cochlear dose, that ultimately dictate hearing outcomes.